1. Field of the Invention
The present invention relates to a radio communication method, a radio transmission apparatus and a radio receiving apparatus using time and frequency diversity.
2. Description of the Related Art
Conventionally, some diversity techniques have been put into practice in the field of radio communication. A diversity is a technique to transmit and receive a plurality of signals by using a plurality of radio communication resources and improve reception quality by choosing a received signal which is in good communication status at the receiving end, or by combining a plurality of received signals. As a type of diversity, there are a time diversity, in which identical signals are transmitted twice at different times, a frequency diversity, in which identical signals are transmitted by two different frequencies, an antenna diversity, in which the transmitted signals are received by two antennas arranged at different locations, or a path diversity, in which a plurality of delayed waves arriving at the antennas via different propagation paths (channels) are combined.
NTT DoCoMo, KDDI, Mitsubishi Electric, NEC, Panasonic and Sharp, “Repetition of ACK/NACK in E-UTRA Uplink”, R1-070101, 3GPP TSG-RAN WG1 Meeting, #47bis (2007.01), (Document 1), disclose a technique combining time diversity and frequency diversity. In Document 1, as shown in FIG. 1, two transmit RF signals which have different center frequencies are generated from an identical data signal (ACK/NACK signal in Document 1) and transmitted at different times. Since the two transmit RF signals have different center frequencies, even in the case where they are transmitted via a channel having frequency selectivity likewise a multipath channel, there will be less possibility that both transmit RF signals will concurrently pass through a frequency band with large power attenuation (frequency diversity). In addition, because the transmitting time of the two transmit RF signals is different, the peak power can be prevented from increasing, which is caused by the transmit RF signals becoming multicarrier signals, and the two transmit RF signals can also be prevented from being transmitted concurrently during a time zone with large power attenuation (time diversity).
However, in the method described in Document 1, frequency conversion must be performed twice in order to transmit the same data signal in different frequencies and at different times. In the frequency conversion, for example, it is necessary to carry out the following processes; (a) generate sinusoidal signals, (b) multiply a transmit baseband signal obtained by modulating the data signal by the sinusoidal signal, and (c) filter the multiplied signal. In the method of Document 1, these processes are performed twice, by using sinusoidal signals having different frequencies.
Generally, the process of this type of frequency conversion requires an increase in calculation amount in accordance with the signal length of the data signal. In the case of a digital signal process, the number of times of multiplication is required in proportion to the signal length. Accordingly, it is not favorable for mobile appliances requiring downsizing, lightness and low power consumption to carry out the frequency conversion process twice, since this leads to increased power consumption and circuit size.